1. Field of the Invention
The present invention relates to a semiconductor device and the method for forming the same. More particularly, the present invention relates to a semiconductor device with two different MOS structures.
2. Description of the Prior Art
Metal-oxide-semiconductor field-effect transistor, MOSFET for short, is a widely used field-effect transistor. According to the polarity of its channel, the MOSFET is divided into the P-type and the N-Type, i.e. PMOS and NMOS.
Functionally speaking, PMOS and NMOS each has different threshold voltages, which are determined by the difference of the work function of the gate and the channel material. This can be accomplished by two different metals as the gate materials.
Because two layers of different metals are required to be the gate material, conventionally the two layers are formed respectively. For example, U.S. Pat. No. 7,074,664 discloses that firstly a first gate electrode material layer is entirely formed on a substrate, later a selective etching is performed based on a well defined patterned hard mask, then a second gate electrode material layer fills the space caused by the selective etching, finally the surfaces of the first gate electrode material layer and the second gate electrode material layer are planarized to complete the fabrication.
In U.S. patent publication 2005/095763, it discloses that a sacrificial layer is entirely formed on a substrate, later the sacrificial layer is selectively removed to be filled with a first gate electrode material layer, then the sacrificial layer is completely removed to be filled with a second gate electrode material layer to complete the fabrication.
No matter which fashion is used, a selective etching must be performed to form different metal layers for respectively deciding the threshold voltages of the PMOS and NMOS. It is clear that the concept of forming the first gate electrode material layer first and followed by the etching to form the second gate electrode is both complex and troublesome and does not meet the demand of simplicity pursued by the industry.
Therefore, it is necessary to provide a simple and convenient method for forming a PMOS and an NMOS with different threshold voltages.